I think that my comments, along with the comments of everyone I knew, were invariably “could do better if he would take things seriously”. I think that if I would add up the number of such comments on my report cards and those of my friends, the total would literally be in the hundreds.
On the other hand, when I was a teacher, I more commonly wrote things like “I’m glad he’s not trying, because if he did, he’d only be disappointed.” And the principal always edited them to say something about a heart of gold.